The problem is the word "cheaper".

For the opposite word, "expensive", it all depends on the word before it: "as expensive" or "more expensive".

"100 widgets cost $100. Unit price is $1." They are now twice _as_ expensive -> they now cost $200. Unit price is now $2.

"100 widgets cost $100. Unit price is $1." They are now two times _more_ expensive -> they now cost $300. Unit price is now $3.

But How do you do this with the word "cheap"? Does "cheaper" clearly mean "3x as cheap" or "3x more cheap" (which even sounds a bit wrong)? I guess it means "more cheap" means "cheaper".

Better to avoid the problem and say "They are now two times the cost, or half the cost". Which is much clearer.